The present invention relates to a pesticide which releases hydrogen phosphide (phosphine) upon reaction of a phosphide with water. In particular, this invention relates to a hydrolyzable metal phosphide composition comprising solid matter, treated with a compound which renders the phosphide free to be reached by and react with water in a gaseous state but renders the composition sufficiently repellent to water in a liquid state, to reduce the likelihood of spontaneous ignition.
There are a number of suitable hydrolyzable phosphides commercially available for pesticidal purposes, such as aluminum phosphide, calcium phosphide, and magnesium phosphide, which produce phosphine upon reaction with water. Unfortunately, these phosphides react more or less violently and exothermically on contact with water, especially in the liquid state, and tend to spontaneously ignite the phosphine created thereby. This spontaneous ignition is undesirable in that it becomes a serious fire hazard to materials and/or structures which are being fumigated by the pesticide.
Many attempts have been made to reduce or completely eliminate the tendency of the pesticide and/or products thereof to spontaneously ignite upon reaction with water. The prior art has used additives, such as metallic soaps, paraffin, resins and waxes, to make metal phosphides hydrophobic. Various silicones have also been used for this hydrophobizing purpose. For example, the Federal Republic of West Germany patent application AS No. 1 023 265 (publication date of Jan. 23, 1958) discloses the use of silicone on a phosphide to retard the gaseous escape of phosphine therefrom when the mixture is exposed to moisture. In practice, such a retardation of the release of phosphine is not desired, since humans will not be able to return to the area being fumigated until all phosphine has been released and has had a chance to dissipate therefrom. In addition, some of the prior art coatings maintain the phosphide in an unhydrolyzed state for such long periods of time that the grain or other material being fumigated could contain hazardous phosphide particles long after the fumigation would supposedly be complete. Thus it is desired to have a hydrolyzable phosphide preparation which reacts completely and with relative speed upon interaction with gaseous moisture but repels liquid water therefrom.
Federal Republic of West Germany patent application OS No. 1 542 877 discussed coating the surface of molded bodies, that is tablets, with methyl or phenyl silicone oils to produce the liquid hydrophobizing effect without interfering or impairing the production of the phosphine. However, the silicone oils used in this manner, do not produce the complete liquid phase water repelling effect necessary to insure that spontaneous ignition does not result even when additions of up to about 3% by weight of methyl or phenyl silicone oil are used. Greater amounts than 3% by weight of the silicone oil considerably impair the flowing and pressing properties of the pesticide mixture and limit further normal processing or use.
The other conventional hydrophobizing agents have similar disadvantages to those discussed above. For example, metallic soaps require relatively large amounts thereof to achieve an adequate hydrophobizing effect upon the phosphide. However, large amounts of the hydrophobizing additive limit the use of and inhibit the effectiveness of other important ingredients which are used for suppressing the spontaneous ignition characteristic of such pesticides. In addition, the flowing and pressing properties of such pesticides are also adversely effected by large quantities of these types of hydrophobizing additives. According to U.S. Pat. No. 3,132,067, solid water repellent coating type additives, such as paraffins, synthetic resins, and waxes, preferably in amounts of 4% by weight, are preferably applied in uniform layers upon the phosphide which is frequently difficult to accomplish and normally an extra pretreatment step is required directed toward such additives, such as grinding, melting, dissolving or the like. According to said U.S. Pat. No. 3,132,067 and its prosecution file, such paraffins, synthetic resins and waxes, function to form a relatively strong waterproof coating or shell about the phosphide which is imprevious to water not only in its liquid, but also in its gaseous state. Before reaction can occur with any moisture in any significant amount, this coating must be broken, which according to that patent is accomplished after a delay of several hours from the moment of exposure to the atmosphere by a "bursting" agent which builds up pressure within the shell, thereby breaking the shell and allowing moisture to enter. Problems arise if the composition does not comply with various critical requirements. If the shell rupture is too wide, such a coating will have the disadvantage of allowing liquid water to enter into contact with the phosphide as soon as the shell is broken. In addition, those particles of phosphide which were coated with the hydrophobizing agent but because of poor mixing or other reasons undergo no opening of the shell, may not react with moisture at all during the appropriate time of fumigation and thereby remain in an unreacted state and, thus, present a potentially hazardous situation since the remaining phosphide shells could conceivably be broken open later, thereby releasing the phosphine in the presence of humans or animals. Pesticides with such shells also take a relatively long time after exposure for the release of the phosphine to be substantially complete, even when all of the shells are broken open. As with the previous conventional hydrophobizing agents or additives, relatively large amounts of such waterproofing coatings are normally required and consequently cause the resultant negative processing effects attendant with such large amounts.